The present invention relates to an improved magnetopermanent plate.
Magnetopermanent plates which anchor metal parts, such as molds or the like, to particular machines in order to produce various products have long been used in various fields.
For example, said plates are often used in the ceramics industry for tilemaking: they are in fact meant to provide the support for the molds in presses, and said support must be as strong and precise as possible, in order to prevent the hammer, by falling onto the mold, from irreparably damaging the entire unit due to an even minimal accidental displacement.
In practice, conventional plates are constituted by a solid body made of ferromagnetic material, in which parallel grooves are provided which are in practice mutually similar slots in which the coils are positioned and fixed; the energization of the coils produces the anchoring effect.
Currently there are substantially two types of said plates: a first one, of the so-called flux-reversal type, in which the neutrality of the surface, which is periodically required for example to replace the molds, is provided by reversing the polarity of some magnetic poles, so as to close the field lines below the active contact surface; and a second one, of the full demagnetization type, in which the demagnetization of the active contact surface is achieved by means of a procedure for demagnetizing the permanent magnets located in the plates in order to bring residual induction to zero.
In both cases, the demagnetization of the magnets occurs by acting on the same coils used for magnetization and therefore for the activation of the attraction force.
These coils are usually constituted by windings of enameled copper wire, which are wrapped in a containment wrapping formed by means of a cotton tape and are inserted in said grooves, inside which they are locked by poured epoxy resin; the grooves are then closed with a brass bar.
The enameled copper wire currently used to form said coils has a maximum operating temperature that is significantly lower than 200.degree. C., while the epoxy resin has a maximum attainable limit of approximately 160.degree. C.
These limits prevent the use of magnetopermanent plates in molds used for the production of items made of thermosetting plastics or elastomers.
These molds must in fact operate at temperatures close to 200.degree. C. in order to allow correct processing of the raw materials; during the heating step, in order to overcome a natural thermal inertia of the metals that constitute said molds, the temperature may exceed the above threshold of 200.degree. C. and be then brought to a steady-state value.
The materials that compose the coils and the epoxy resins that retain them in the grooves collapse at these temperatures: the coils burn and the resins soften, losing their ability to provide mechanical retention.
This makes the plate unusable.
Additionally, another drawback which is typical of magnetopermanent plates, particularly of the type that uses flux reversal for demagnetization, resides in the fact that some of the magnetopermanent materials used, such as rare earths or ferrites, have high polarization temperature coefficients and this causes a decrease in the attraction force as the operating temperature increases.
Finally, magnetopermanent plates meant for use in machine tools must be impermeable to the emulsified water usually used for machining.
The penetration of said emulsion (water and oil) in the coil seats causes aggression of the insulation of the wire that composes said coils, which are accordingly damaged, thus making the plate unusable.